Localization of catalase was effected in human leukocytes with 3,3' diaminobenzidine tetrahydrochloride (DAB) using the procedure described by Novikoff and Goldfischer. Catalase was found in large cytoplasmic granules of neurophils which coincided with primary (azurophilic) granules. Acatalasemic leukocytes were used to check the specificity of the cytochemical procedure. In acatalasemic neutrophils no evidence of DAB reaction (pH 9.0) was observed in the granules while normal neutrophils showed intense reaction at this pH. In contrast the DAB reaction conducted at pH 7.2 was distinctly positive for myeloperoxidase in both acatalasemic and normal neutrophils. Sucrose gradient centrifugation will be employed to separate the various granules of the leukocytes. The precise relationships of these granules to catalase and myeloperoxidase will be sought.